The cancer drug discovery field is experiencing unprecedented revolution accompanied by growing excitement from researchers, drug developers, patients and investors, partly due to recent clinical success of cancer immunotherapy. Human immune defense system comprises both innate and adaptive immune pathways. All the targets drugged by the recently approved cancer immunotherapeutic agents including the immune checkpoint proteins PD-1, PD-L1 and CTLA-4 function in adaptive immune pathways. In contrast, targets involved in the innate immune pathway have not matured to regulatory approval for systemic use though several candidates are now in preclinical and clinical development. Drugs targeting innate immunity represent great opportunity for more rapid and broader spectrum anti-cancer effect than adaptive immunity. Furthermore, combinations of drugs targeting innate and adaptive immunity are expected to produce strong synergistic efficacy owing to their complementary nature as body’s immune defense. Toll-like receptors (TLRs) are a family of proteins that recognize pathogen associated molecular patterns (PAMPs). Their primary function is to activate innate immune responses while also involved in facilitating adaptive immune responses. Different TLRs exert distinct functions by activating varied immune cascades. TLR8 is more broadly expressed in immune cells than other TLRs such as TLR7 and TLR9. One of the major causes of cancer immunotherapy failure is potent suppression of immune response by Treg cells. TLR8 is the only TLR that has been shown to be necessary and sufficient to reverse the suppressive function of Treg cells leading to strong tumor inhibition. Therefore, agonists targeting TLR8 are expected to be effective cancer therapy. However, there is no approved TLR8 selective agonist at present. There is only one TLR8 selective agonist in clinical development. Through structure-based drug design, we discovered a novel, highly potent and selective small molecule TLR8 agonist, DN-A1. DN-A1 exhibited strong in vitro cellular activity with EC50 at 4.23 nM, about 30-fold more potent than the only drug candidate in clinical trials. The activity was highly selective for TLR8 over other TLRs. DN-A1 displayed superior in vitro ADMET and in vivo PK profiles. DN-A1 showed clean CYP profile with IC50 over 10 μM for all major CYP isoenzymes tested including 3A4, 1A2, 2C9, 2C19 and 2D6. DN-A1 had favorable hERG parameter with IC50 over 30 μM whereas the reference compound’s hERG IC50 was 3.84 μM suggesting potential cardiac toxicity. DN-A1 significantly impeded tumor growth as a single agent and was well-tolerated in mouse tumor models. Taken together, DN-A1 warrants further development as a potential best-in-class preclinical drug candidate for TLR8-mediated cancer immunotherapy.
Citation Format: Yuxun Wang, Heping Yang, Longsheng Wang, Guangliang Fu, Fang Bao, Fang Liu, Shuwen Ren, Huanping Li, Haixia Ji, Yajun Yu, Zhiheng Wu, Panhu Zhu, Hui Xu, Yaqiao Gao, Pei Wang, Shoujun Chen, Daxin Gao. Development of a novel highly potent TLR8 selective agonist for cancer immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2995. doi:10.1158/1538-7445.AM2017-2995